The soda sticker will be placed on each person's cruise card/room key. It costs $3.50 per day plus 15% gratuity. For a typical 7 day cruise the card cost $28.18. Each person needs their own card.
To save even more, don't get the insulated mug they try to sell you with the soda sticker. It really isn't all that great and doesn't hold that much soda. Your cruise card with the sticker is what gets you the drinks, so you really don't need to use the travel mug.
Also, if you like a large soda, ask for a large cup when you get your drink. The bartender ill give you a medium or small unless you specify a large.
Have a GREAT Cruise!